Field of the Invention
This invention relates to the development of a safe interior pressure in order to affect the seal of flexible borehole liners in boreholes in geologic formations with shallow water tables or formations exhibiting artesian hydraulic head conditions. The liner may or may not be installed in the borehole by pressurized eversion.
Background Art
A “borehole” is a hole, e.g., a drilled shaft, into the Earth's subsurface. The hydraulic conductivity profiling techniques described in my U.S. Pat. Nos. 6,910,374 and 7,281,422, whose teachings are incorporated herein by reference, have been used in over 300 boreholes since 2007. These patents describe a hydraulic transmissivity profiling technique which carefully measures the eversion of a flexible borehole liner into an open, stable, borehole.
Liners are usually installed into the open boreholes using a water level inside the liner which is significantly higher than the water table in the formation penetrated by the borehole. However, when such a desirable excess head is not available within the borehole, a scaffold and an extension of the surface casing, above the ground's surface, are often used to obtain the higher water level within the liner to achieve the pressure head needed during the installation of the liner. The continued presence of a scaffold and/or casing extension is usually an objectionable means for providing long-term pressurization of the flexible liner adequately to seal the borehole. In other situations, the inventor of the invention hereinafter disclosed has filled the liner with a weighted mud to achieve an interior pressure sufficient to seal the liner against the artesian head in the formation. In a deep borehole, however, the mud density required to seal the borehole throughout an upper portion of the hole produces such a high pressure at the bottom of the borehole as to risk bursting of the borehole liner. The present invention ameliorates or eliminates the risk of busting the liner, while yet promoting a reliable sealing of the borehole by the liner.
With the foregoing background, the presently disclosed invention was developed.